Cleaning (de-poisining) PEMFC electrodes from strongly adsorbed species on the catalyst surface

ABSTRACT

A method for cleaning the electrochemical catalyst of fuel cell electrodes that is performed by applying a power pulse, using a low-power supply, across the fuel cell electrodes. The power pulse removes chemisorbed chemical species from the electrochemical catalyst of the electrodes.

RELATED APPLICATIONS

This application claims the benefit of provisional application No.60/679,038 filed on May 6, 2005, titled “Cleaning (De-poisoning) PEMFCElectrodes from Strongly Adsorbed Species on the Catalyst Surface”.

STATEMENT REGARDING FEDERAL RIGHTS

This invention was made with government support under Contract No.W-7450-ENG-36 awarded by the U.S. Department of Energy. The governmenthas certain rights in the invention.

FIELD OF THE INVENTION

The present invention relates generally to fuel cells, and, moreparticularly to a method for cleaning (de-poisoning) fuel cellelectrodes from strongly adsorbed species on the catalyst surface.

BACKGROUND OF THE INVENTION

Proton Exchange Membranes Fuel Cells (PEMFC) are devices that generateelectrical power from two complementary electrochemical reactions.Hydrogen is oxidized at the anode and oxygen is reduced at the cathode.Thus, efficient fuel cell operation relies on the availability of boththe cleanest fuel and air possible. These reactions take place on thesurface of highly dispersed Pt catalysts. The catalytic activity of thePt surface is very sensitive to the presence of certain impurities.Therefore, PEMFC performance may be strongly affected by the presence ofcontaminants in the fuel and in the air stream. In the hydrogen fuel,the impurities can be present in the primary source of fuel or can begenerated during the reforming process. For instance, reformation ofhydrocarbon fuels such as methane or gasoline, besides H₂, may producevarious impurities at levels that can be detrimental to fuel cell (FC)operation. Typical fuel impurities are carbon monoxide (CO), ammonia(NH₃) and hydrogen sulfide (H₂S).

Contaminant species, such as hydrogen sulfide, poisons Pt catalystsirreversibly. That is, a neat (impurity-free) hydrogen stream will notbe able to clean a sulfur-poisoned Pt surface because of the highchemical affinity of H₂S with metals. FIG. 1, shows cyclic voltammograms(CV) of a fuel cell anode fully poisoned with H₂S. Two major features inthis CV indicate the presence of sulfur species chemisorbed onto the Ptsurface. Within the potential domain 0.1 to 0.4 V, in the first cyclethe typical peaks of a clean Pt catalyst corresponding to H-desorptionare totally absent because the active sites are blocked by sulfurspecies. The second feature is seen in the potential range 0.9 to 1.3 V,which appears as two major merging oxidation waves. These currentscorrespond to the electrochemical oxidation of chemisorbed sulfur tonon-poisoning species. Subsequent cycles become similar to that obtainedon a clean Pt surface. Consequently, FC performance after cleaning bycyclic voltammetry is the same as observed before contamination withH₂S.

Other impurities may be present as contaminants in the ambient airinjected to the cathode during FC operation. For instance sulfur dioxide(SO₂) is a common air pollutant that comes from fossil fuel combustionand is particularly abundant in urban areas. Depending on theconcentration, SO₂ presence in the FC cathode air stream may have fastand irreversible negative effects on FC performance.

The CV in FIG. 2 shows similar features to those described in FIG. 1 forH₂S, suggesting that the chemisorbed sulfur species are similar in bothcases. Again, FC performance after cleaning by cyclic voltammetry is thesame as that obtained before the cathode was contaminated with sulfurdioxide. The facts described above, advise that electrode contaminationwith either hydrogen sulfide or sulfur dioxide should be avoided by allmeans, and there is a clear need to address what to do if the catalyst(electrode) becomes inadvertently poisoned with one of thesecontaminants.

Cyclic voltammetry is an electroanalytical technique that not onlyprovides information about the status of the surface of the Pt-catalyst,but also by applying it to a poisoned Pt-catalyst electrode results infull electrode cleaning. However, performing CV involves interruptingfuel cell operation for a considerable amount of time (at least 1 hour).In addition the electrode being probed has to be purged with an inertgas (N₂ or Ar), which is time consuming and requiring a potentiostat,which is a rather expensive instrument.

Additional objects, advantages and novel features of the invention willbe set forth in part in the description which follows, and in part willbecome apparent to those skilled in the art upon examination of thefollowing or may be learned by practice of the invention. The objectsand advantages of the invention may be realized and attained by means ofthe instrumentalities and combinations particularly pointed out in theappended claims.

SUMMARY OF THE INVENTION

In accordance with the purposes of the present invention, as embodiedand broadly described herein, the present invention includes a methodfor cleaning the electrochemical catalyst of fuel cell electrodes thatis performed by applying a power pulse, using a low-power supply, acrossthe fuel cell electrodes. The power pulse removes chemisorbed chemicalspecies from the electrochemical catalyst of the electrodes.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and form a part ofthe specification, illustrate the embodiment(s) of the present inventionand, together with the description, serve to explain the principles ofthe invention. In the drawings:

FIG. 1 is a graph of cyclic voltammograms of an anode Pt catalyst afterfull poisoning with H₂S. A curve before poisoning is also shown forreference.

FIG. 2 is a graph of cyclic voltammograms (1) of a fuel cell cathode Ptcatalyst after full poisoning with SO₂.

FIG. 3 a is a schematic of the electrical connections from the powersupply to the fuel cell electrodes, for electrochemical cleaning of thecatalyst for cathode cleaning.

FIG. 3 b is a schematic of the electrical connections from the powersupply to the fuel cell electrodes, for electrochemical cleaning of thecatalyst for anode cleaning.

FIG. 4 is a graph of the fuel cell current density transient recordedbefore, during, and after poisoning the FC cathode with 10 ppm of sulfurdioxide in the air stream. The plot also includes the result of the 5 spower pulse cleaning stage.

FIG. 5 is a graph of the current-voltage power pulse applied to a fuelcell poisoned with sulfur dioxide. The power supply was set at 2.0 A and1.4 V and the result of this 5 s power pulse application on cellperformance is also shown in FIG. 4.

FIG. 6 is a graph of the fuel cell current density transient recordedbefore, during, and after poisoning the FC anode with 2 ppm of hydrogensulfide in the H₂ fuel stream. The hydrogen flow was interrupted priorto pulsing for 10 minutes. The plot also includes the result of the 20 spower pulse cleaning stage.

FIG. 7 is a graph of the fuel cell current density transient recordedbefore, during and after poisoning the FC anode with 2 ppm of hydrogensulfide in the H₂ fuel stream. The hydrogen flow was uninterrupted. Theplot also includes the result of the 20 s power pulse cleaning stage.

FIG. 8 is a graph of the current-voltage power pulse applied to a fuelcell poisoned with hydrogen sulfide. The power supply was set at 30 Aand 1.4 V and the result of this 20 s power pulse application on cellperformance is also shown in FIG. 7.

DETAILED DESCRIPTION

The present invention comprises a method for in-situ cleaning fuel cellelectrodes whose electrochemical catalyst is poisoned with stronglychemisorbed chemical species. Example 1 below shows the techniqueapplied to a SO₂-poisoned cathode Pt-catalyst. The procedure is alsoapplicable to other chemical species that chemisorb on Pt-catalysts,other metals or alloys used as electrochemical catalysts, independent oftheir origin. These species include but are not limited to H₂S, HCN,olefins and aromatic compounds.

The method consists of applying a power pulse, for 1 to 20 seconds,using a low power supply (0.5 to 6.0 W/cm²), across the fuel cellelectrodes. Because the adsorbed species on the catalyst surface usuallyis a monolayer of molecules, the total amount of electrical chargenecessary for the electrochemical desorption of these species is small.Inherently, the power requirements are also small. A shortvoltage/current pulse is enough for cleaning the contaminated catalystsurface. Low and high limits on the applied voltage are imposed, thelower limit to ensure that the electrochemical desorption processoccurs, and, the higher limit to avoid electrochemical reactions thatmay irreversibly damage the electrode materials. In a preferredembodiment the voltage limit is from 1.2 to 1.4 V. However, in anotherembodiment, larger currents (4.5 A/cm² or 6 W/cm²) may be used.

Cleaning an anode poisoned with H₂S, can be carried out using either ofthe following two options; a) with interruption of the H₂ flow prior topulsing, which requires low currents (up to 0.4 A/cm²) and b) withoutinterruption of H₂ flow, which requires high currents (up to 4.5 A/cm²).In this instance a large portion of the current is used in H₂ oxidation.In both cases, the voltage must be kept below 1.4 V for reasonsmentioned above.

EXAMPLE 1 Cleaning a Fuel Cell Cathode Contaminated with SO₂

Poisoning with SO₂

FIG. 4 shows the cell current density as a function of time for a fuelcell experiment in which the cell operated at constant voltage (0.6 V).Initially the cell cathode ran on impurity-free air for 20 minutes andthen operated with air contaminated with 10 ppm of SO₂ for 20 minutes.The negative effect of the impurity on performance was observed as soonas the SO₂ injection started and it is indicated by the sudden decreasein the current, which eventually dropped below 20% of the originalvalue. Once the SO₂ injection was interrupted, the cell ran on neat airagain for about 24 minutes. A slow and small recovery was observed.Numerous SO₂ poisoning tests indicate that the recovery does not improveeven if the cell continued operating on clean air for several days.

Fuel Cell Cathode Cleaning

The fuel cell was momentarily turned off before the cleaning wasstarted. Then, the positive terminal of the power supply was connectedto the fuel cathode and the negative terminal to the fuel anode, asshown schematically in FIG. 3 a. A power pulse was applied for 5seconds. The power supply was fixed at 1.4 V and the current wasrecorded as a function of time as shown in FIG. 5. Immediately after thepulse, the cell was disconnected from the power supply and turned on. Asshown in FIG. 4, the recovery of the fuel cell performance was quitefast and the cell current practically returned to the original valuerecorded prior to poisoning.

EXAMPLE 2 Cleaning a FC Anode Contaminated with H₂S

Anode Poisoning with H₂S and Cleaning Electrode with H₂ FlowInterruption

FIG. 6 shows a similar experiment to example 1, but this time for ananode whose hydrogen fuel supply was contaminated with 2 ppm of H₂S.Initially the cell ran on impurity-free hydrogen for 40 minutes, showingsteady performance. Then it was exposed to H₂S-contaminated hydrogen for30 minutes. The cell current dropped considerably to 56% of its initialvalue. As expected, after stopping the injection of H₂S the anodeexperienced insignificant recovery when the cell resumed operation onneat hydrogen again. Prior to applying the power pulse to the FC, the H₂gas flow was completely interrupted while the air flow was significantlyreduced for about 10 minutes. This allowed the existing hydrogen at theanode to be consumed, leaving only the chemisorbed electroactive specieson the Pt catalyst to be electro-oxidized by the external power pulse.Thus, prior consumption of H₂ to pulsing reduces the power requirementsfrom the power supply.

After most of the hydrogen at the anode was consumed, the cell wasmomentarily turned off and the positive terminal of the power supply wasconnected to the fuel cell anode and the negative terminal to the fuelcell cathode (see FIG. 3 b). Then, the power supply was set at a fixedvoltage of 1.4 V and a power pulse was applied to the cell for 20 s.Notice that in this example, the power supply terminals are connectedopposite to the previous one. In this case the cell recovered 95% of theinitial fuel cell current prior to poisoning.

Anode Poisoning with H₂S and Cleaning Electrode with H₂ Flow:

FIG. 7 shows a similar experiment to that of FIG. 6, except for amodification in the cleaning procedure. First, the cell ran onimpurity-free H₂ for 50 minutes. Then, the cell continued running on H₂contaminated with 2 ppm of H₂S for 40 minutes. As a result, theperformance of the cell decreased to 40% of the initial value and didnot recover even after again running on impurity-free H₂ for another 30minutes.

Then the cell H₂ flow was decreased from 160 to 80 sccm (standard cubiccentimeters per minute) and a 20 second electrical pulse was appliedwith the power supply settings at 1.4 V and 15 A. After the pulseapplication, the cell performance recovery was fast and complete, asindicated by comparison of the initial and final current values.

Full cell performance recovery is the main advantage of the proceduredescribed. It appears that by keeping the fuel flowing during theapplied pulse, the desorbed active sulfur-species washes away from theanode catalyst. Without fuel flowing, some sulfur-species may re-adsorbon the catalyst surface resulting in a partial catalyst cleaning andperformance recovery. However, as explained above this option requireshigher power because most of the supplied current is used in oxidizingthe H₂ fuel. As shown in FIG. 8, only for a small fraction of the timethe pulse reaches values above 0.9 V, which is the minimum voltage forinitiating the catalyst cleaning.

These two examples demonstrate a simple cleaning method for reactivatingfuel cell electrodes irreversibly poisoned with strongly chemisorbedspecies. These kinds of impurities can fully disable a fuel celloperation in short exposure times. The worse aspect of this ordeal isthe irreversibility of the process. Once the catalyst is poisonedfurther operation with neat fuel in the anode or clean air in thecathode does not recover the original performance.

The injection of small amounts of air, a proven approach to increasinganode CO tolerance (Gottesfeld, U.S. Pat. No. 4,910,099), is notefficient in the case of H₂S poisoning due to the high potentialrequired for electro-oxidation of the impurity.

The present technique provides: simplicity of application, low cost ofthe required equipment, short time length of the procedure, norequirement for inert gases, and minimal interruption of the fuel celloperation.

The foregoing description of the invention has been presented forpurposes of illustration and description and is not intended to beexhaustive or to limit the invention to the precise form disclosed, andobviously many modifications and variations are possible in light of theabove teaching.

The embodiments were chosen and described in order to best explain theprinciples of the invention and its practical application to therebyenable others skilled in the art to best utilize the invention invarious embodiments and with various modifications as are suited to theparticular use contemplated. It is intended that the scope of theinvention be defined by the claims appended hereto.

1. A method for cleaning the electrochemical catalyst of fuel cellelectrodes, comprising: applying a power pulse using a low-power supplyacross said fuel cell electrodes for a period of time sufficient toremove chemisorbed chemical species from said electrochemical catalyst.2. A method of removing a contaminant from a fuel cell cathode,comprising: connecting a fuel cell cathode to a positive terminal,connecting a fuel cell anode to a negative terminal, and applying afixed voltage low-power power supply across said fuel cell cathode for apredetermined period where said contaminant is removed from said fuelcell cathode.
 3. The method of claim 2 where said fixed voltage rangesfrom about 1.2 to 1.4 volts.
 4. The method of claim 2 where saidpredetermined period ranges from about 1 to 20 seconds.
 5. The method ofclaim 2 where said low-power supply ranges from about 0.5 to 6.0 W/cm².6. The method of claim 2 where said fixed voltage is applied for a timesufficient to restore said fuel cell current to within 95 to 100% ofinitial current.
 7. A method of removing a contaminant from a fuel cellanode, comprising: connecting a fuel cell cathode to a negativeterminal, connecting a fuel cell anode to a positive terminal, andapplying a fixed voltage across said fuel cell anode for a predeterminedperiod where said contaminant is removed from said fuel cell anode. 8.The method of claim 7 where said fixed voltage ranges from about 1.2 to1.4 volts.
 9. The method of claim 7 where said predetermined periodranges from about 1 to 20 seconds.
 10. The method of claim 2 where saidlow-power supply ranges from about 0.5 to 6.0 W/cm².
 11. The method ofclaim 7 where said fixed voltage is applied for a time sufficient torestore said fuel cell current to within 95 to 100% of initial current.